1. Field of the Invention
The invention relates to a fluid passage structure of an internal combustion engine which enables fluid to flow through the interiors of a cylinder block and a cylinder head.
2. Description of the Related Art
Inside a cylinder head and a cylinder block of an internal combustion engine, fluid passages through which fluid including oil such as lubricant, coolant and the like flow are formed. In many of such internal combustion engines, as disclosed in Japanese Patent Application Laid-Open No. 63-303266, an in-block flow passage as a fluid passage formed in a cylinder block and an in-head flow passage as a fluid passage formed in a cylinder head are coupled to each other on an abutment plane defined by a bottom face of the cylinder head and a top face of the cylinder block. Thus, fluid flow between the cylinder block and the cylinder head.
In such a fluid passage structure of an internal combustion engine, an opening position of an in-block flow passage on a top face of a cylinder block needs to coincide with an opening position of an in-head flow passage on a bottom face of a cylinder head so as to ensure that the in-block flow passage communicates with the in-head flow passage. However, since the cylinder block and the cylinder head are complicated in structure, the degree of freedom in arranging the in-block flow passage and the in-head flow passage is low, and it is not easy to design the fluid passage structure such that the opening positions of the flow passages coincide with each other. Also, due to such a restriction on arrangement of the flow passages, it is sometimes inevitable to form the in-block flow passage and the in-head flow passage obliquely with respect to the top face of the cylinder block and the bottom face of the cylinder head respectively. As a result, for example, oblique holes need to be drilled. This constitutes a factor which makes it difficult to manufacture an internal combustion engine having the fluid passage structure as described above.
In addition, such a fluid passage structure of an internal combustion engine may be susceptible to a problem that will be described below. In a fluid passage structure as described above, it is sometimes required that the flow rate of fluids flowing between a cylinder block and a cylinder head be restricted. The flow rate can be restricted by adjusting flow areas of an in-block flow passage and an in-head flow passage. However, if those flow areas are made smaller than a certain area, elongated holes of a great length need to be drilled, for example. This makes it difficult to form the in-block flow passage and the in-head flow passage. For example, as shown in FIG. 12, it is also contemplable to mount an in-block flow passage 191 or an in-head flow passage 192 (the in-block flow passage 191 in an example illustrated in FIG. 12) with an orifice 194 in which an elongated hole 193 is formed, and to restrict the flow rate of fluids by throttling part of the flow passage. In such a case, however, the orifice 194 needs to be prepared as a separate piece. As a result, an increase in manufacturing cost is ineludible.
Further, according to the fluid passage structure of the internal combustion engine disclosed in the aforementioned patent document, as shown in FIG. 13, a communication hole 204 of a head gasket interposed between an in-block flow passage 201 and an in-head flow passage 202 is formed to be small in diameter, so that the communication hole 204 substantially acts as a throttle for restricting the flow rate of fluid. With this arrangement, the flow rate is restricted without increasing the number of parts used. However, considering the fact that the head gasket 203 has a thin flat shape, it is feared that the peripheral portion of the communication hole 204 will deform due to the fluid flow pressure applied thereto, as represented by the dashed line in FIG. 13. In particular, if adopted as a flow passage delivering an oil, such as a lubricating oil, the flow pressure applied to the peripheral portion aforementioned may become as high as 1 MPa, for example, during a cold start of the engine where the viscosity of the oil is still high. For this reason, the above-described structure makes it difficult to maintain a sufficient durability of head gasket 203.
Thus, none of fluid passage structures developed or proposed so far enables to favorably restrict the flow rate without causing problems, such as a reduction in the durability of the head gasket as described above.